This invention relates to the field of vacuum microelectronic devices and their manufacture. In particular, in one embodiment the present invention provides an improved method and structure for a vacuum microtriode device using standard semiconductor fabrication techniques.
Vacuum tube and integrated circuit devices and their fabrication have been well known for many years. Recently, techniques originally used for fabrication of integrated circuit devices have been applied to make miniaturized vacuum tube devices. This new technology is referred to as "vacuum microelectronics." Vacuum microelectronic devices offer several advantages over traditional integrated circuit devices. Since a vacuum is an ideal electron transport medium, electrons travel at a higher speed increasing the device switching speed. Also, since there is no scattering medium to impede electron transport, there is no heat produced as in traditional integrated circuits. An additional advantage of vacuum microelectronic devices is their relative temperature and radiation insensitivity compared to traditional integrated circuit devices. Also, since no active junction regions exist, there is no associated parasitic capacitance and the semiconductor medium used for processing vacuum microelectronic devices does not need to be as of high a quality as used in traditional integrated circuit devices, decreasing manufacturing costs.
Although several types of vacuum microelectronic device structures and processing methods have been proposed, no proposed method or structure has resulted in a high density, easy to manufacture structure. As of this date vacuum microelectronic devices are not generally commercially available.
In particular, processes currently being utilized to produce vacuum microelectronic devices have been plagued by process control problems. For example, the process described in the article "Development Toward The Fabrication of Vacuum Microelectronic Devices Using Conventional Semiconductor Processing" by Zimmerman et al. describes an inverted wedge emitter structure fabricated from a cusping mold. The emitter structure is produced by depositing silicon dioxide of precise thickness into an etched cavity of particular dimensions. Cusp formation is dependent on the thickness of the deposited silicon dioxide and the aperture depth and width. Because a small change in the aperture depth and width results in a large change in thickness of the silicon dioxide layer at the cusp apex, controlling the cusp formation and characteristics of the cusp is difficult. The aforementioned paper reported processing problems and did not report fully functional results at the time of publication.
The paper "Field-Emitter Arrays for Vacuum Microelectronics" by C. A. Spindt et al., and U.S. Pat. No. 4,721,885, "Very High Speed Integrated Microelectronic Tubes," to Brodie both describe field-emitter arrays with molybdenum cones as emitters and molybdenum gates. The molybdenum cones and gates are formed by sputtering molybdenum on a substrate. Although these references report functional devices, the devices are operational only at relatively high voltages (&gt;20 volts). The devices described in the Spindt article and the Brodie patent do not readily lend themselves to the low voltage operation used in current VSLI or ULSI devices. High voltage potential in the prior devices is required due to the relatively large emitter tip radius' (200 .ANG.-500 .ANG.) produced by currently reported microelectronic triode devices. In addition, these devices do not lend themselves to large scale integration of components as is common in planar semiconductor technology.
A microtriode device having a reduced emitter tip radius which is easy to manufacture is needed to produce vacuum microelectronic devices operable at voltages below 10 V.